The present invention relates to a display sheet having a layer which can change states to provide a viewable image.
Currently, information is displayed using assembled sheets of paper carrying permanent inks or displayed on electronically modulated surfaces such as cathode ray displays or liquid crystal displays. Other sheet materials can carry magnetically writable areas to carry ticketing or financial information, however magnetically written data is not visible.
A structure is disclosed in PCT/WO 97/04398, entitled xe2x80x9cElectronic Book With Multiple Display Pagesxe2x80x9d which is a thorough recitation of the art of thin, electronically written display technologies. Disclosed is the assembling of multiple display sheets that are bound into a xe2x80x9cbookxe2x80x9d, each sheet is arranged to be individually addressed. The patent recites prior art in forming thin, electronically written pages, including flexible sheets, image modulating material formed from a bi-stable liquid crystal system, and thin metallic conductor lines on each page.
Fabrication of flexible, electronically written display sheets are disclosed in U.S. Pat. No. 4,435,047. A first sheet has transparent ITO conductive areas and a second sheet has electrically conductive inks printed on display areas. The sheets can be glass, but in practice have been formed of Mylar polyester. A dispersion of liquid crystal material in a binder is coated on the first sheet, and the second sheet is bonded to the liquid crystal material. Electrical potential applied to opposing conductive areas operate on the liquid crystal material to expose display areas. The display uses nematic liquid crystal material which ceases to present an image when de-energized.
U.S. Pat. No. 5,223,959 discloses a plurality of polymer dispersed liquid crystal material, each having a different dye material of red, green, or blue dye material. Differing electrical signals to common electrodes operate on each of the materials to control the state of each type of dyed liquid crystal material. The patent requires the use of conventional nematic liquid crystals with a dye to absorb light. The droplets are chemically treated to be stable in either a clear or a light absorbing state. The invention also requires materials having different response times to electrical signals. The device must be continually driven so that the human eye perceives complementary colors. This arrangement has the disadvantage of requiring continuous, high speed electrical drive because the materials do not maintain their state. The material must be driven to achieve a neutral color density.
U.S. Pat. No. 5,437,811 discloses a light-modulating cell having a polymer dispersed chiral nematic liquid crystal. The chiral nematic liquid crystal has the property of being driven between a planar state reflecting a specific visible wavelength of light and a light scattering focal-conic state. Said structure has the capacity of maintaining one of the given states in the absence of an electric field.
U.S. Pat. No. 3,816,786 discloses droplets of cholesteric liquid crystal in a polymer matrix responsive to an electric field. The electrodes in the patent can be transparent or non-transparent and formed of various metals or graphite. It is disclosed that one electrode must be light absorbing and it is suggested that the light absorbing electrode be prepared from paints containing conductive material such as carbon.
U.S. Pat. No. 5,289,301 discusses forming a conductive layer over a liquid crystal coating to form a second conductor. The description of the preferred embodiment discloses indium tin oxide (ITO) over a liquid crystal dispersion to create a transparent electrode.
Current state of the art discloses the need for a second conductor over a polymer dispersed liquid crystal material. In particular, cholesteric materials require one of the two conductors to be light absorbing and conductive. Materials have been proposed for the application including carbon or metal oxides to create a black and conductive surface for polymer dispersed cholesteric liquid crystal materials.
It is an object of this invention to provide a display sheet having a light absorbing and electrically conductive electrode for cholesteric liquid crystals.
It is another object of the invention to provide a the electrically conductive electrode which is light absorbing, highly conductive and durable.
It is a further object of the invention to provide an effective, rapid method of forming said light absorbing, electrically conductive electrode for use in a display sheet.
These objects are achieved in a display sheet having polymer dispersed liquid crystals, comprising:
a) a substrate;
b) a state changing layer disposed over the substrate and defining first and second surfaces, such state changing layer having the polymer dispersed liquid crystals having first and second optical states, which can change state;
c) a first transparent conductor disposed on the first surface of the state changing layer; and
d) a second conductor on the second surface of the state changing layer and having a composite structure of at least two layers of different materials wherein the composite structure absorbs light and is electrically conductive so that when a field is applied between the first and second conductors, the liquid crystals change state.
The present invention provides for first and second conductors that maximize light absorption and electrical conductivity of the second conductor. Materials are disclosed that can be deposited in a single multi-station vacuum evaporator to provide a low-cost, durable and conductive surface.